With this change, we now have ~1200 CellAllocators across both LibJS and
LibWeb in a normal WebContent instance.
This gives us a minimum heap size of 4.7 MiB in the scenario where we
only have one cell allocated per type. Of course, in practice there will
be many more of each type, so the effective overhead is quite a bit
smaller than that in practice.
I left a few types unconverted to this mechanism because I got tired of
doing this. :^)
This makes XHR now rely on Fetch, which allows it to correct send
Origin and Referer headers, CORS-preflight and filtering and many other
goodies.
The main thing that's missing is Streams, which means we can't properly
produce progress events or switch to the Loading ready state.
This also doesn't implement the Document responseType just yet.
